Work chairs should conform with their body supporting surfaces to the different seating positions of the user. Thereto, there have been many proposals. Thus, the seat and backrest, for example, are divided into several parts and hingedly or adjustably connected with one another. As the user, during the day, frequently changes his seating position, the chair mechanism is also provided so that it automatically conforms to the position of the user.
There are basically three seating positions. In the forwardly-bent seating position for writing or for reading from material lying on the table top, the back is inclined forwardly. In the upright seating position, in particular in the use of machines, the back stands approximately vertical. In the relaxed or listening position, for example when telephoning or speaking, it is suitable for the back to be inclined rearwardly. These three seating positions should be assume without problems and optimal support should be provided for the respective body parts.
Especially in the rearwardly inclined relaxing, or listening position, the seat surface should no longer lie horizontal because otherwise a slipping is to be feared and this is opposed by friction of the seat material whereby, however, slipping between the clothing the body of the user can occur. Therefore, it is desirable for the seat to be inclined rearwardly. It is sought to effect the change to these different positions by suitable supporting and adjusting mechanism of the chair in order to achieve a so-called dynamic seating. Thereby, an articulated connection between the seat and beackrest is provided as for example according to DE-OS No. 33 22 450 or DE-OS No. 30 36 993 whereby in the first case an articulated construction with a fixed pivot axis, and in the second case a bendable connection between the seat and the backrest is provided. A plurality of other construction are also known. They differ essentially in the precise position of the point of articulation and the length of the lever of the linkage in order to achieve the proper relationship between the parts. Many constructions seek to position the pivot axis of the different support surfaces so as to conform with the pivot points of the user's joints. In many cases, this is not achieved and hence in many support and movement lapses, there are anatomically unfavorable support and movement relationships.
In the forwardly inclined writing position, the back as a rule has no contact with the backrest. Upon backward movement, such contact first begins approximately upon reaching the upright machine operating position. It has been shown that it is advantageous to maintain the support between the lower lumbar vertebra and the pelvis in an unaltered angle position and to adapt the movement of the kinematic to this relationship. Such construction is known basically from DE-GM No. 77 21 954. Thereby are the seat and backrest formed of three parts whereby the seat part is divided into a flat part and a rear wedge part. The flat part and wedge part are connected with one another through a horizontal transverse axis. The wedge part and the backrest can be fast with one another or can be connected with one another through an elastic connection. The supporting kinematics are so selected that upon the backrest being pressed rearwardly against a self-blocking air-spring, the angle between the flat part and the wedge part of the seat part is increased and the forward region of the flat part of the seat supporting the upper thigh is raised while the wedge part as a whole is lowered and to an almost horizontal end position. The flat part is thereby supported by a fixed articulation axis lying in the region of the articulation axis between the flat part and the wedge part and a probably slidable support in approximately the middle region of the flat on a seesaw. The seesaw in turn is linked on the chair frame far forwardly of the articulation axis of the flat part and has a seesawlike support lever extending under the other articulation axis on which the wedge part is fixed. The seesaw is provided on the rear end in the region between the wedge part and the back part with an articulated support on a self-blocking air-spring. The self-blocking air-spring in turn is arranged forwardly and downwardly inclined and is there linked to the frame. This mechanism requires a plurality of toggle joints with six pivot axes or pivot regions. Moreover, this kinematic gives quite a special movement and is selected so that the forward region of the flat part of the seat is raised upon tilting rearwardly. The feet are thereby lifted from the floor. A kinematic of this kind is not suitable for all three work positions of the chair. Various other efforts show that the forward-most seat region is maintained at a constant height and that therefore a linkage and/or sliding support is provided there, for example according to DE-OS No. 33 22 450 or DE-GM No. 78 15 561.
The construction according to DE-GM No. 77 21 954 also lacks a construction which makes possible a long sitting in the forwardly bent handwriting position with which namely the seating surface is inclined forwardly slightly downwardly. It is thus not suitable for the practical requirements for which it is important that the seat and backrest optimally conform to the various positions of use of persons working in different manners.
With the construction according to DE-OS No. 33 22 450 and DE-GM 78 18 561 as well as with many other constructions, there is a backrest carrier which is linked rigidly to the frame in the region of the column and on which the seat is linked in variable angle positions to the backrest whereby upon leaning back, there is a deviation between the pelvis and the spinal column which is undesirable. The linked support between the rear surface of the seat according to DE-GM No. 78 15 561 at best enables a stretch position between the two seat surface parts, however, no further lowering of the rear end of the seat which opposes the natural support of the forces in the rearwardly leaning position.
With the construction according to DE-OS No. 30 36 993 there is support of the seat and backrest with only few linked axes and a linkage coupling between the seat and backrest which works together with the few supports. However, these supports are provided outside the seat and hence not suitable for many chair construction. There is also a forwardly leaning arrangement which does not lift the thigh on leaning back but changes the angular position between the pelvis and spinal column upon leaning back in a manner that is undesired by many users and it requires a spring linkage construction between the seat and backrest. The inclination of the seat surface is constantly connected with a corresponding inclination of the backrest which is likewise undesired for many users or use situations. From DE-GM No. 76 12 629, there is known an arrangement for adjusting the seat inclination with which the seat can be adjusted in its inclination with respect to the foot frame with a curved disc arrangement. The backrest on its part is arranged in an adjustable position on the foot frame. They are not coupled with one another. This chair is not suitable for a dynamic seating. It shows, however, how with an inclination adjusting cam disc and a link axis, there can be realized a suitable inclined position in which at the same time the cam disc construction is immobilized. For a direct use in a support mechanism for a dynamic seat and backrest support, this arrangement however is not suitable.